United States patent U.S. Pat. No. 5,540,549 relates to a fluid damper device comprising a driving element which is constituted by an internal member 22 possessing an axial direction end driven by a drive element which includes a moving element 23 and a ball joint 65 intended to compensate in particular for cocking motion of the helicopter blade and also for twisting motion thereof due to pitch control.
Damping is provided firstly by a laminated structure 28 of alternating rigid layers 28 and elastomer layers 32, and secondly by a piston 38 which moves in a fluid chamber. The laminated structure has high stiffness parallel to the above-mentioned axial direction and low stiffness in a direction perpendicular to the axial direction.
The piston 38 serves to obtain vibration damping over the entire range of vibration amplitude by simultaneously implementing two related phenomena, firstly a "throttling" phenomenon of headless through a constriction, and secondly a viscous braking phenomenon, i.e. damping by shear in a viscous oil, with the oil used being a silicone oil having a viscosity lying in the range 10,000 centistokes to 100,000 centistokes (1 stoke=10.sup.-4 m.sup.2 /s).
That device suffers from the following drawbacks:
firstly, the throttling phenomenon predominates when high amplitude damping forces are required, which is the case in the intended application to helicopter rotors; and PA1 secondly, it does not escape from the effects on the device of the compression vibrations that take place with helicopter rotors. Such compression vibrations affect the fatigue characteristics of the system. It may be observed that the ball joint 65 serves to act only on torsion and cocking motion but that it does not avoid the compression phenomenon.
The first above-mentioned drawback gives rise to poor linearity in the damper device which is due to the fact that damping by throttling is naturally non-linear since it varies with the square of displacement speed.
The second drawback identified by the present Applicant is accompanied in above-mentioned patent U.S. Pat. No. 5,540,549 by a parasitic phenomenon which induces modulation of the damping by viscous braking. That damping depends to a large extent on the clearance that exists between the piston 38 and the walls of the chamber, and also on the exact axial direction of the piston. Since the effect of the compression phenomenon tends to alter the direction of the piston axis and/or to move it off-center, there results a damping modulation phenomenon which adds to the non-linearity of said damping by throttling.